To test the function of and to calibrate gas sensors, the measured gas to be detected or a substitute gas that appears to be suitable is, in general, admitted to the gas sensors at fixed time intervals. It is either possible to use this test gas in compressed gas containers together with suitable gas admission means, for example, with pressure reducers, or to generate the test gas directly and to apply it to the sensor through suitable gas admission devices. The use of compressed gas containers with corresponding means is complicated and requires corresponding logistics and handling.
No commercially available gas generators which can generate the test gas CO are currently available for testing the function of carbon monoxide sensors. Only the generation of CO with the use of carbon electrodes by means of electrolytic oxygen generation and subsequent reaction of the carbon electrode itself to form a mixture of CO and CO2 is described in US 2003 014 5644, but it found no application in a commercial product. The reason for this might be the drawbacks of the use of such consuming electrodes, because sufficient long-term stability of a generator cannot be attained with such a design and the reaction is strongly affected by the climatic conditions prevailing at the time of the testing, such as temperature and relative humidity, as a result of which greatly fluctuating CO contents will be obtained in the test gas.
Substitute gas calibration with hydrogen (H2) is therefore frequently used as an alternative, as is described, for example, in WO/99 24 826, but this does not make it possible to directly infer the sensitivity of the gas sensor to be tested to the primary test gas, because the reaction of H2 may very well be successful on a poisoned catalytic material of a corresponding electrochemical gas sensor that is consequently no longer suitable for measurement, whereas the same catalytic material does not react CO electrochemically any longer, i.e., the calibration is incorrect.